The main objective of the research program is to analyze thalamocortical relations with neuroanatomical techniques. The afferent and efferent connections of two thalamic regions, in particular, the auditory and the visual, are being analyzed using the methods of anterograde degeneration, autoradiographic analysis of axonally transported tritiated compounds, and retrograde axonal transport of horseradish peroxidase. The goal of these experiments is to analyze the subdivisions in these regions of the thalamus in terms of similarities and differences in their patterns of connections. The organization of the synaptic complexes in two of these subdivisions, the dorsal lateral geniculate and the pulvinar nucleus, are being analyzed using electron microscopic techniques. A number of different species including iguanas, tree shrews, squirrels and monkeys are being compared in order to determine how these regions of the thalamus and cortex vary among different species.